A projector apparatus is required to uniformly project an image with sufficient color rendering property to a rectangle screen. Therefore, 0.15 mg/mm3 of mercury is enclosed in an arc tube of a light source for such a projector apparatus, and a short arc type discharge lamp (hereinafter also referred to merely a lamp) in which the mercury vapor pressure in the arc tube turns into 150 or more atmospheric pressure at time of lighting, is used therefor.
FIG. 14 is a schematic view of the structure of a conventional short arc type discharge lamp. In an arc tube thereof, 0.15 mg/mm3 of mercury and rare gas containing halogen are enclosed. A pair of electrodes 2 made of tungsten is arranged to face each other therein. Sealing portions 3 are formed, extending from the arc tube. Metallic foils 4 are buried in the respective sealing portions 4. External leads 5 are connected to the respective metallic foils 4. Each of the electrodes 2 is formed by winding a coil around a rod member made of tungsten, and then melting only a tip side portion of the coil. That is, a tip side portion of the electrode is formed as a lump by melting the coil, and a back end side thereof stays in the coil shape. This kind of electrodes is disclosed in Japanese Patent Application Publication No. 2005-19262.
In a short arc type discharge lamp having such electrodes, electric discharge changes from mercury arc discharge, to glow discharge, and from the glow discharge to arc discharge in that order. Description thereof will be given below referring to FIG. 14. A first stage of the electric discharge is the mercury arc discharge which begins at the mercury adhering to the electrodes 2 and 2. Although this mercury arc discharge is maintained until the mercury adhering to the electrodes is blown away therefrom, the electrodes 2 and 2 are not heated to a temperature sufficient for thermionic emission in the mercury arc discharge. In addition, there is no such an initial stage of the electric discharge in case of a short arc type discharge lamp which does not contain mercury. A second stage of the electric discharge is the glow discharge. During this glow discharge, these electrodes are heated by collisions of cations of rare gas and those of mercury. When these electrodes 2 are heated to the temperature at which thermionic emission becomes possible by the glow discharge, the electric discharge shifts from the glow discharge to arc discharge. When these electrodes include portions which tend to be easily heated at the time of glow discharge, these portions are preferentially heated so as to emit thermal electrons, whereby it is possible to smoothly make transition from the glow discharge to the arc discharge. Coil portions 2a are formed on the electrodes 2 shown in an FIG. 14, in order to make the portions heated preferentially. The third stage of the electric discharge is the arc discharge. Since the tip portions of the electrodes 2 are lower in temperature than the coil portions 2a immediately after the electric discharge shifts to the arc discharge, a coil arc discharge which begins at these coil portions 2a is performed. After the temperature of the tip portions of these electrodes 2 rises due to heat transfer from the respective coil portions 2a, the arc discharge which begins at the tip portions of the electrode 2 is performed.